A membrane can be defined as a barrier separating two fluids, which barrier prevents hydrodynamic flow therethrough, so that transport between the fluids is by sorption and diffusion. The driving force for transport through the membrane is pressure, concentration or a combination of both. During operation permeate molecules dissolve into such a membrane at its upstream surface followed by molecular diffusion down its concentration gradient to the downstream face of the membrane. At the downstream face of the membrane the permeate is evaporated or dissolved into the adjacent fluid phase. The property of the membrane describing the rate of transport is called its permeability.
The importance of membranes in chemical technology for separating liquid and/or gaseous components from one another is rapidly growing, since the membrane permeation process is particularly useful as a separation technique whenever conventional separation methods cannot be used economically to get reasonable separation. Separation by means of membranes has a further advantage in that the components to be separated are not subjected to thermal loads and not changed in chemical structure.
Membranes can be distinguished as to their microstructural forms in porous ones and non-porous or dense ones. Membranes are usually nominated as porous when they contain voids that are large in comparison with their molecular dimensions of permeates. Transport of permeates occurs within the pores of such membranes. Porous membranes have high transport rates which, however, is accompanied with a very poor selectivity for small molecules, and therefore less suitable for gas separation techniques.
Dense membranes, on the other hand, have the ability to transport species selectively and are therefore applicable for molecular separation processes, such as gas purification. With such dense membranes, even molecules of exactly the same size can be separated when their solubilities and/or diffusivities in the membrane differ significantly. A problem with dense membranes is the normally very slow transport rates. To attain acceptable transport rates, required for commercial application in separation processes where productivity is of paramount concern, it is necessary to make such membranes ultrathin. This can be construed from the following equation applicable for gas separation ##EQU1## wherein N represents the permeation rate,
P is the permeability, i.e. product of solubility and diffusivity, PA1 (p.sub.1 -p.sub.2) is the pressure difference over the membrane, and PA1 L is the membrane thickness.
Similar equations are known for solid/liquid, liquid/liquid and gas/liquid separation by means of dense membranes
From the above it will be clear that the rate of permeation per unit surface for a given material of the membrane and a given permeate depends upon the thickness of the membrane.
Various techniques are known for producing very thin membranes. The most common methods are melt extrusion, calendering and solvent casting. Melt extrusion should be carried out with rather complex equipment and it sets requirements, among others, thermal stability, to the material to be extruded. Calendering doe not permit the production of membranes with a thickness less than about 50 .mu.m. A more preferred production method is solvent casting, which involves forming a solution of the membrane material, normally consisting of a polymer, and casting it onto a liquid substrate to produce a thin liquid film which is dried so that a solid membrane film is formed. To provide mechanical strength to the membrane, the film is normally arranged on a porous substrate, which may have any suitable configuration such as a flat plate or a hollow fiber. In another more preferred production method, a membrane is formed by plasma polymerization on a porous substrate. Plasma polymerization is a process wherein organic monomers are introduced into a space filled with a plasma, whereby the organic monomers are activated for example by applying an electric field and are converted into radicals or ions to effect polymerization. Membranes comprising one or more layers of plasma polymerizate can be made to have a very high selectivity. The permeability of such a membrane is, however, very poor. The porous substrate may be a sintered material, woven or nonwoven fibers, or a porous polymer film.
With the above known methods dense membranes can be produced with a very small thickness of only some nanometers. Although the membrane films can be so produced that they have a high selectivity, essential for a proper gas separation process, in combination with a reasonable permeability rate, essential from economical point of view, the porous substrate, necessary for giving the membrane film sufficient mechanical strength, forms a serious impairment of the permeability of the whole membrane structure. The pores in the porous substrate should be sufficiently small so that there is no risk of the membrane film tending to sag into or rupture adjacent to these pores during the use of the membrane. For supporting a membrane film with a thickness of about 0.1 .mu.m, the porous substrate pores should be not greater than about 0.5 .mu.m. If the pores are however very small, they will tend to impede flow through the porous substrate merely as a result of their size.
The amount of permeation through a membrane with a given composition does not only depend upon the thickness of the membrane but also upon its area. If a membrane film is arranged on a porous substrate, the area for gas transport is determined by the total area of the pores at the surface of the substrate and is therefore substantially smaller than the area of the membrane film.
It has been proposed to facilitate movement of water molecules from the surface of a water-absorbing reverse osmosis membrane film into a porous substrate layer by interposing a hydrogel layer capable of absorbing a substantial amount of water between the membrane film and the substrate layer. The thickness of such a hydrogel layer may vary considerably, depending on its liquid (usually water) content, which is likely to pose problems with regard to the continuity of the membrane film which is attached to the hydrogel layer and with regard to the performance of the membrane system in the absence of liquids.